The binding energy of electrons in a metal is 250KJ/mol.What is the threshold frequency of metal ?

Question

$\begin{array}{1 1}(a)\;6\times 10^{14}sec^{-1}&(b)\;7\times 10^{14}sec^{-1}\\(c)\;6\times 10^{-10}sec^{-1}&(d)\;4\times 10^{14}sec^{-1}\end{array}$